The cooking time for food in a microwave oven presently is arrived at by various means. If large pieces of meat are being prepared, such as a roast, turkey, etc., one may insert a meat thermometer in the food item and continue cooking until the desired meat temperature is reached. The meat thermometer may be coupled to the microwave source through a microprocessor to deactivate the microwave source at the desired temperature. Alternatively, one may follow the oven manufacturer's recommendations relative to cooking time as a function of the type of food being cooked and the size of the item. In the case of vegetables, eggs, beverages, etc., thermometers or temperature probes are impractical, and thus the user must rely on the oven manufacturer's recommendation as to cooking time. However, the cooking time is a function of the power absorbed in the food, which typically can vary by plus or minus 270 watts due to variations in line voltage, oven production, etc. In a worst case situation the absorbed power can vary by plus or minus 360 watts.
Since the microwave oven is capable of cooking and heating a variety of food items, it is thus desirable to include means for automatically cooking or heating all food items until they have reached the desired final temperatures without the need for temperature probes. One such proposed automatic cooking system involves sensing the change in humidity in the microwave oven cavity as the food is being heated, determining when a humidity "trip point" is reached, and relating that factor to the total cooking time. The disadvantage of such a system is that it is strongly dependent upon the food's initial temperature and surface area, thus the system potentially can be only typically 20 to 35% accurate.
Another proposed system involves having the consumer enter information concerning the food mass and category into a microprocessor controlled microwave oven. The information is used to address a look-up table which contains the cooking instructions for that particular food. The look-up table is essentially a stored cookbook. One disadvantage of this system is that a meat temperature probe must be used with the look-up table, but the primary disadvantage is low resolution. That is, a finite amount of information can be stored in a look-up table in the microprocessor and in order to avoids excessive costs, some cooking situations must be left out. As an example, if a look-up table were designed to contain the information stored in a standard microwave cookbook, approximately 42,768 bytes of 8 byte microprocessor memory will be required. This amount of memory reduces the number of microprocessors that can be used for the oven controller, and the addition of more memory capacity could increase the cost of the controller excessively. Thus, the resolution of the look-up table would have to be reduced to a practical level, and as the resolution is decreased the system error is correspondingly increased, so that at a practical level the system error would be from 25 to 50%.